CUET · PHYSICS · PYQ PAPER 2025

A solenoid with an iron core and a bulb are connected to an a.c. source. The bulb is glowing with certain brightness. When the iron core is removed from the solenoid, the brightness of the bulb

A increases
B decreases
C remains same
D increases and later decreases

Verified Solution

Answer & Solution

Correct Answer

(A) increases

Step-by-step Solution

Detailed explanation

Removing iron core decreases inductance \( L \). Inductive reactance \( X_L = 2\pi f L \) decreases. Impedance \( Z = \sqrt{R^2 + X_L^2} \) decreases. Current \( I = V/Z \) increases. Brightness of the bulb increases.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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Metal	Work Function (eV)
Na	1.92
K	2.15
Ca	3.20
Mo	4.17